My 2002 1500 Sliverado 4.8L all of sudden is hard to start, needs much cranking, and has a rough low idle. I have no engine light so not sure if any codes are in the computer. Seems to be worse when the truck is warmed up? (Seems opposite of what I'd expect for a problem like this?) The truck has 125k miles. I replaced plugs/wires and air filter, no difference. Went to replace the fuel filter but fittings are too badly rusted so will have to most likely need to cut out filter and replace some tubing so was going to put that off. Other than the starting and rough idle the truck runs great! No missing or hesitating at acceleration or constant speed? I've put the key to the ON position for several seconds before starting thinking if it was a fuel pump or blocked filter this would make a difference but it doesn't? Any ideas out there?

I have a 2003 with the same engine and it just started doing the exact same thing a week ago. I changed the fuel filter and am running injector cleaner but doesn't seem to be helping. I talked to a few guys last night and these symptoms may be the result of a bad fuel pressure regulator. It is usually on the left side of the engine on the fuel rail. If there is fuel in the vacume line that attaches to it then it could be bad. Another thing to check is the mass airflow sensor. The wires may need cleaned or may be broken.

It really Should have seta code if the O2 sensor was bad. Not arguing with anyone, just saying. And as was mentioned it sure wouldnt hurt to clean that MAF sensor.

The problem though could be caused by something as simple as a vacuum leak.

Ive personally seen where some vacuum leaks dont show up till things warm up for a bit and the hoses I guess are a tad more pliable causing them to sag, spread open a little and then leak.

You might try and take a look at good look at all vacuum hoses, try and spray a little carb cleaner if you like on the hoses and if the engine momentarily speeds up youve found a leak.

Honestly, Ive seen many rough idle complaints caused by a simple leaking vacuum hose.This would also go along with your "runs great at speed" as vac leaks tend to REALLY rear their ugly heads at idle.

Hey, It's wortha shot, wont cost much and it eliminates a possible problem.

Id really change that fuel filter if its been on that long. It may even be partially plugged. Id definitely change it before I even considered checking fuel pressure as a possible problem.A dirty filter is also very hard on the fuel pump itself as the pump is trying hard to maintain pressure but it has restriction making it work harder than it should.

Well, "Service Engine" light finally came on and the idle got so bad I had to take it in to my local shop as I just didn't have time to mess with it myself. Turned out to be the fuel pressure regulator. The regulator diaphram was leaking and making the mixture extremely rich.

Final update on my problem. Turns out about a month after I had the fuel pressure req replaced I started having the exact same symptoms again! Took the vehicle back to the same shop as I just assumed it was a bad fuel pressure req that was installed, but NO! Turned out to be a leaking intake manifold gasket and a bad O2 sensor. Was told that the O2 sensor was most likely burned out due to the leaking gasket. After a total of about $1000 in repair bills the truck is finally running ok. To answer the question about where the fuel pressure req is: on my truck it is on the driver side fuel rail directly over the valve cover.

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